This invention relates to cable heads such as are used in suspending well logging tools at the end of a "wireline" or cable in the borehole of an oil or gas well, in order to determine various parameters associated with the oil and/or gas production potential of formations intersected by the borehole.
A cable head provides for the mechanical and electrical connection of the logging cable to the uppermost tool in the logging tool string. The mechanical connection of the cable head serves three principal functions: first, to securely fasten the logging cable to the female portion of the electrical connection between the cable and the uppermost logging tool; second, to maintain the integrity of the male to female electrical connection; and third, to ensure the failure of the mechanical connection at a predetermined tensile load level. This latter function is important because it is fairly common for cable-supported tools to become stuck in boreholes. The provision of a specific failure point at the cable head permits the operator to preserve the majority of the cable in the event the tool string becomes hopelessly stuck in the borehole. The inclusion of a failure point in the cable head having a lower tensile strength than the cable is generally referred to as providing a "weak-point". It should be noted, however, that a weak point must be strong enough to support the static weight of the tool string suspended from the cable as well as any additional loads imparted to the tool during normal operations or in reasonable efforts to free the tool string should it become stuck Accordingly, the required predetermined tensile failure point of the weak-point is primarily a function of logging depth. Therefore, the weak-point component is offered to the logging operator in several tensile strengths in order to accommodate the needs of logging to different borehole depths. It is, of course, desirable to provide separation of the cable from the tool string at the cable head not only to retrieve as much of the cable as possible, but to avoid a loose skein of cable on top of the cable head, the presence of which would hamper subsequent efforts to "fish" out the tool string by engaging it with a fishing tool such as is known in the prior art.
Known cable heads employing weak point elements suffer numerous disadvantages. For example, the prior art devices are unreliable in their failure points; difficult to inspect to ensure that the weak point element has not been partially stressed even though it remains unfailed and thus will be unpredictable in failure point; hard to replace as they are inside the cable head, requiring disassembly; and cannot be used with perforating guns as the shock from the shaped charge firing may cause unwanted failure of the weak point.